This invention relates to electron emission devices. In particular, the invention is concerned with high efficiency semiconductor cold cathodes which employ new P-N combinations of materials to form heterojunctions.
Considerable work has been done in the field of cold cathodes. Avalanche-type cold cathodes have been constructed of silicon. A discussion of this work is reported in Philips Technical Review, VOL. 43, No. 3, Jan. 1989 by G.G.P. van Gorkom and A.M.E. Hoeberechts. Unfortunately, silicon cathodes suffer from poor efficiency, e.g., .about.1%. Such cathodes reverse bias a semiconductor junction into avalanche and because the excitons, i.e., hole-electron pairs, are generated at varying locations and in a random process, the electrons reaching the surface of the cathode have a bell-shaped energy distribution of which only the tail end of the bell exceeds the work function of silicon and are emitted.
Various schemes have been proposed to use GaAs, AlAs, and alloys thereof to accelerate electrons to energies exceeding the work function of GaAlAs or cesiated GaAlAs. These approaches, unfortunately, have severe limitations. The cesiated surfaces are extremely fragile in that cesium is a highly reactive element and even minuscule exposure to virtually any contaminant will poison the surface. GaAs has a work function of 4.7 eV necessitating very high "hot" energies of electrons within the GaAs. These high energies cause the upper conduction band valleys to be populated and in so doing, the electrons become heavy and slow down thus greatly defeating the purpose of this approach. The bare GaAlAs surface is also easily poisoned by contaminants.
Diamond is thought to have a negative electron affinity surface and has been shown by M. Kordesch of Ohio University to act as a cold cathode as disclosed in Electronics Letters , 1 Aug. 91, VOL. 27, No. 16. Diamond has the added advantage that its surface, once terminated by hydrogen, is extremely stable and virtually impossible to contaminate. Unfortunately, there are no known impurities with which to reliably and controllably render diamond N-type; damage-induced N-type. activity in diamond is virtually impossible to adequately control.
In experiments by the applicant and others, AlN (aluminum nitride) has recently been shown to exhibit a negative electron affinity surface. AlN and SiC (silicon carbide) have recently been shown to form miscible alloys over the entire mole fraction compositional range and heterojunctions of AlN and SiC have also been demonstrated as have heterojunctions with AlN and AlSiCN. While N-type impurity doping of AlN is difficult because the material must be absolutely free of oxygen, it is possible. A good discussion of some recent work with SiC-AlN mixtures is contained in the "3rd International Conference on Amorphous and Crystalline and Other Group IV-IV Materials", APR 11-13, 1990, G. L. Harris, et al. Additional details regarding it are found in U.S. Pat. No. 4,382,837.